With all due respect to the UMR authors, I think the location of a decap on a set of power planes DOES matter. There is an inherent time delay associated with the current flow from the location where a decap is placed to the location where the chip to bypassed is located. The decaps job is to provide current to the load quickly enough that a voltage collapse on the planes doesn't occur. If the decap is too far away from the current consuming load it is possible that load's need for a pulse of current will be over before the current can get there. At low frequencies you can place a decap just about anywhere on a plane and it will function just fine. At higher frequencies the decap must be closer to the device to be bypassed. The concept of "effective bypassing radius" says that a decap must be within say 1/6 to 1/10 wavelength at the frequency of interest to be effective. (the exact number is debatable). The frequency of interest is the resonant frequency of the mounted decap (where the caps ESR is lowest), and the distance is a function of the speed of propagation with a given dielectric between the planes (around 180ps/inch for FR4). You need to consider the role each component of a PDS (power distribution system) plays in maintaining a low impedance across a wide bandwidth. A typical PDS is composed of a VRM, bulk capacitors, ceramic decaps, and the power planes and last but not least the package housing the silicon. The VRM provides a low impedance from DC to perhaps a few hundred kHz, the bulk capacitors contribute their impedance profile from a few hundred kHz up to about a MHz. From a Mhz to maybe 120MHz or so the ceramic decoupling caps provide low impedance. Above that frequency the distributed capacitance provided by the planes provides the requisite low impedance to the PDS. When the individual impedance profiles of each of the constituent parts are superimposed, the resulting composite profile provides a low impedance over the full frequency range that the PDS was designed for. Note that above the package resonance frequency (which usually is from about 50 to 100 MHz for typical packages) decoupling on the board is ineffective and the necessary decoupling must be provided for either in the package or on the silicon. However, at frequencies above the package resonance frequency the discrete decaps and distributed plane capacitance can be most useful for EMI purposes. To summarize, at low frequencies, the decoupling capacitors can be placed almost anywhere withing reason and will be effective. As the frequency of interest increases, the allowable distance decreases. As an example, at 1 MHz the wavelength (on FR4) is around 214 meters. So lambda/10 is around 21 meters. Hence, if your decap which is effective at 1 MHz is within 21 meters of the consuming device, then the decoupling will be effective. Conversely, if you are trying to decouple at device with a decap that is effective at say 100 MHz then the wavelength of interest is about 1.4 meters. Lambda/10 is then .14 meters (approx 5.5 inches). In this case if the decap is appreciably more than 5.5 inches from the currewnt consumer then the delay will be such that the decap will not be able to provide current quickly enough to be effective. -Ray Anderson Sun Microsystems Inc. Lee Ritchey wrote: >> If you remember the UMR paper on power bus decoupling, it made a clear case >> that the location of the capacitors is relatively unimportant. Abe Riazi wrote: >To be fair, it should be added that different recommendations ( from above) exist in literature >regarding optimum location of the decoupling caps. > >Two examples follow: > >1. Stephen H.Hall et al., " High-Speed Digital System Design........", on >page 247 it is stated: > >" Figure 10.10 suggests not only that the local decoupling capacitor value should be chosen >appropriately but that the capacitor is placed physically close to the component to minimize >the loop area ". > >2. A technical paper by R. Chen, "Where to Place Decoupling Capacitors?..." available on >www.sigrity.com web site, implies that the best location for decoupling caps needs to >be ascertained with aid of electromagnetic field simulation. > >Best Regards, > >Abe Riazi >ServerWorks ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu